Stephen R. Leone University of Southern California Los Angeles, 90007
Applications of Lasers to Chemical Research
The laser has brought about a revolution in the basic sciences, especially chemistry. There are few scientific disciplines that have not been affected by lasers and their remarkable possibilities for research. Lasers are heing used to measure earth movements, to look for gravity waves, and to determine distances to the moon's surface with extreme precision. Lasers have vast potential in medicine for surgical healing of retinal damage and bleeding ulcers. Lasers provide the most accurate standards for measuring distance and frequency. Lasers already have many industrial applications, such as precision cutting, drilling, and welding. Laser amplified communications are making rapid advances. Although the laser has not made a direct impact on many chemical industries, it has had a tremendous effect on basic research in the fields of spectroscopy, studies of excited states, and selective chemical reactions. The time is very near when the use of lasers in chemistry will have a dramatic effect on our economy and the energy resources we can utilize. A laser is a source of light, but a very special one. The usual light observed from a flame or light bulb consists of photons emitted in random phase and direction from the electronicallv excited atoms or molecules. This process is called spontaneous emission. Laser light is based on the Drocess of stimulated emission (LASER: lieht amplification by stimulated emission of radiation). stimulated emission occurs when a photon of light hits an excited molecule or atom and causes it to emit a photon with the same frequency and direction as the first ( I ). The laser medium is highly excited, so it can amplify the light by multiple stimulated emissions. The lasing material must then have a population inversion, so that more molecules can emit than absorb. Population inversions are achieved by clever selection of the lasing species and the conditions for excitation (2). One such scheme is shown in Figure 1. Some type of excitation (often hv lieht. - . chemical. or electrical enerev) is used to raise a large number of the atoms or molecules from the ground level 1to some excited level 2. If there is a rapid relaxation from state 2 to state 3, an inverted population can be obtained between levels 3 and 4. If other parameters are suitable, stimulated emission or laser radiation may he observed between these levels 3 and 4 until the population inversion is depleted. Figure 2 shows the basic components of a laser device. The laser medium, shown here as a gas, may be a solid or liquid as well. The medium is excited, in this case by an electric discharge, to achieve the population inversion. The discharge tube has good quality windows set a t Brewster's angle (2) so that there is no reflection loss for vertically polarized laser light. Two mirrors provide feedback, by reflection, of the stimulated emission to amplify the light with the available excited states in the laser medium. Useful laser light is taken out of one of the mirrors by making it partially transmitting. The mirror arrangement is called an
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This mnrcrinl war presented at the Sixleenth Annual Summer Cunferrncr of thc C a l h m i a Arswialiun of Chem~stryTeachers, hriu at .\silon,nr. C n l ~ t o r n m Aucusl . 18 LA. 1971.
[RAPID
.a: : : . .
EXCITATION (light, elecfricol,
LASER
RELAXATION
II
inversion
malecvler RELAXATION Figure 1. Four-level energy diagram for achieving an inverted population and laser action.
LASER TUBE Figure 2. Basic components of a laser, an excited laser medium, and an aptical resonator.
optical resonator, and i t gives the laser its unique small diameter beam quality. Only those photons which propagate precisely along the resonator axis will be highly amplified. This results in an extremely collimated, intense beam of photons, all with the same phase. Lasers for many types of laboratory research are readily available now. These include a wide variety of pulsed and continuous (CW) models with fixed wavelengths all through the ultraviolet, visible, and infrared. Some lasers, such as organic dye lasers, are tunable over large ranges. A variety of new devices, called nonlinear optical devices, may he used with other lasers to produce new frequency ranges for spectroscopy, kinetic studies, and selective photochemistry. The properties of various laser light sources far surpass those of ordinary light sources. Laser light is of extremely narrow line width because the lasing occurs usually on single atomic or molecular transitions. Single resonances of the mirror cavity may be selected which are as narrow as l o + cm-'. Laser lieht is hiehlv - .collimated. I t may he sent " over long distances with little loss, or focused to power densities much hieher than ordinary thermal sources. Power densities as high as 1019 W/cm2 are capable of producing a whole new range of spectroscopic and chemical effects. Pulsed lasers as short as s have been achieved. They allow new dimensions in the study of ultrafast processes in gases and liquids. Spectroscopy with Lasers Perhaps one of the first uses of lasers to come into practicality was in Raman spectroscopy (3).A sample is irradiated with an intense, visible laser beam as in Figure 3. Light scattered at right angles to the beam is detected in a Volume 53, Number 1. January 1976 1 13
SAMPLE
R LASER-
m
L
E
N
s
Figure 3.Schematic of an apparatus for taking Raman spectra.
Figure 4. Apparatus for twa-photon absorption spectroscopy
monochromator with a photomultiplier tube. Most of the light scattered is a t the precise frequency of the laser excitine line. However, some of the ohotons are shifted in the linear Raman effect to higher &d lower frequencies by an amount equal to the enerw of vibrations in the molecules. Raman transitions have zifferent selection rules than the usual infrared absorptions and provide complementary information on vibrational bands which cannot he observed in absorption. With the introduction of very high powers easily available in lasers, Raman spectroscopy has hlossomed as a most fruitful field of research for assigning complex spectra and molecular structure. The narrow line width of lasers allows much better resolution of spectra than previously possible with mercury lamps. Since the laser beam is easily focused to small diameters, microliter samples of compounds may be used. With high powered pulsed and tunable CW lasers, a variety of new nonlinear Raman ohenomena have been discovered. These provide informabon about such things as the orientation effects of macromolecules i m ~ o r tantin photobiology. A second spectroscopic innovation made possible with lasers is two-photon absorption spectroscopy (4). This type of spectroscopy is able to identify and assign molecular and atomic states which are not accessihle to the usual onephoton absorptions. Such states can often play an important part in the excited state chemistry of the molecule. The simultaneous absorption of two visible photons in a single transition is a very weak effect and can he ohserved only with the high light intensities available from lasers. Fieure 4 shows one tvoe of two- hot on absorotion set-un. A continuous probe laser is used 'to provide onk of the tons and a tunable.. hieh-intensitv nuked laser for the other. The probe laser passes tb;oigh the sample and through a partially transmitting mirror to a detector. The pulsed laser passes through the cell in the opposite direction. The two-photon absorption is observed as a dip in the power of the probe laser when the pulsed laser is tuned to the correct wavelength. The symmetries of the excited states can he learned by polarization studies of the two beams. This field of investigation is rapidly growing. I t will be possible to find and describe numerous new electronic states. 14 I
Journal of Chemical Education
A third major contribution of the lasers t o spectoscopy is in high resolution spectra. The energy of ordinary light sources in very narrow bandwidths is too weak to obtain ultrahigh resolution spectra. A single-frequency laser designed for spectroscopy can often be 0.001 A wide. This is much less than the Doppler line width in gas absorptions (a broadenine which arises from velocitv shifts). With the high intensity, narrow line widths of lasers, an ingenious techniaue has been devised to eliminate the Doooler width and see the true ahsorption lines in higher reso&ion. The technique, called saturation spectroscopy ( 5 ) ,is able to locate the precise wavelengths of the absorption lines which are ordinarily masked by Doppler broadening. This method has already given direct measurements of fundamental splitting constants in atoms. I t is being applied eagerly in the infrared and visible to unravel the complex high resolution spectra of such simple molecules as CHq and NOZ. where many aspects of the vibration-rotation and electronic states are not understood. Laser Excited Kinetics The application of lasers to the study of chemical kinetics has bad an enormous effect on the types of phenomena which can he studied. Figure 5 shows a typical diatomic potential diagram. The heavy arrow indicates absorption of laser light into a single excited electronic-vibration-rotation state. A transition like this could occur in the visible or uv, or there could also be excitation into higher vibrational levels of the ground electronic state with an infrared laser. Several thines can occur followine ~ u l s e dor durine continuous excitatyon. A progression of bands may be observed in fluorescence to the ground state. Precise values of molecular constants, dissociation energies, and the turning points of molecular ~otentialscan be obtained from transitions not ordinari1;accessible in absorption. Energy transfer or vibrational relaxation may occur, as indicated by the short arrows. The initially excited single state is depleted as others receive its energy. If a pulsed laser is used and the time behavior of fluorescence detected with a photomultiplier, the lifetimes of the excited states may be measured. Lasers with short pulses (a few nanoseconds) allow very short lifetimes t o be determined accurately. Lifetimes for quenching with other gases
--
I
",
FLUORESCENCE
I RFigure 5. Potential plot of a simple diatomic molecule, energy versus internuClear SeDaratiOn.
RECORDER
-,
URANIUM SCOURCE
LASER -FLUORESCENCE CELL
Hg LAMP 2100-3100
Figure 7. Photoionization set-up for waoium iootope separation.
\
Figure 6. Schematic of an apparatus u s d for measving energy transfer and relaxation rates.
may he determined. In some cases, lifetimes for molecular decomposition are observed. This occurs frequently when a potential curve, such as the dotted one in Figure 5, crosses a hound state. The perturbation a t the crossing point provides a decomposition (predissociation) path for the electronicallv excited hound state. The study of molecular energy transfer was opened wide by laser excitation tools (6, 7). Figure 6 shows a typical experimental apparatus for measuring relaxation and energy transfer rates of the vihrational levels in the ground electronic state. A molecular laser, or other pulsed laser, is used to excite a soecific state in the gas sample. A sensitive infrared detector is used t o monitor the t&e behavior of the excited states in the gas after the laser pulse. Excited vihrational states fluoresce in the infrared, hut only weakly. Therefore good signals can he seen only with high energy laser nulses. - ~-- r~ A large numher of interesting results have been ohtained (7). I t is found that vibrational energy interconverts rapidly among the vihrations of large molecules. Vibrational energy is transferred from the vihrations of one molecule to another rapidly. Deactivation of vihrational energy, however, for say HCI (u = l) hy rare gases (Ar), is comparatively slow. The transfer of such a large excess of energy stored in the vibrational mode into the translational kinetic energy of the HCI and Ar is not favorable. In potentially reactive collisions. however. such as HCI (V = 1) deactivated by C1 atoms, the deactivation is many orders of magnitude more raoid. As a result of these laser-excited measurements, ;better understanding of molecular dynamics is oossible. Theoretical interpretations of the experimental data give good insight into the nature of the molecular forces that cause energy transfer and chemical reaction. Light pulses from picosecond (10-l2 s) lasers are now used to measure relaxation phenomena in liquids (8). The light pulse itself is only about 0.3 mm long, so usual electronic detection schemes cannot he used. But by light delaying and photographic techniques, experiments have been performed on the orientational relaxation of large molecules in solution. Picosecond laser spectroscopy will provide a wealth of information on solution chemistry. I t may answer questions about important photobiological processes such as photosynthesis, where many of the energy transfer events occur on picosecond time scales.
REACTOR
Figure 8. Highly simplified sketch ot laser imploobn fusion reactor.
~
~~~
Laser Induced Chemistry
Lasers have been used to initiate new and unusual chemical reactions (9) and to produce isotopically selective ones (10).For example, if a laser in the infrared is used to excite NzFa vibrationally, i t is possihle to carry out an almost explosive reaction with NO to give NOF and Nz. If the same mixture is simply heated, however, there is no reaction with NO. Instead the NzF4 decomposes to NF3 and Nz. A high energy pulse from a laser is able to excite the vihrations of the gas strongly in a time scale short compared to ordinary
heating. This results in some interesting new chemical effects. Numerous experiments demonstrate the use of lasers to carry out isotopically selective reactions and isotope separation (11). In some examples, such as formaldehyde, it is possihle to excite with a uv laser one isotope (e.g. H2I3CO) in a mixture of 12C and 13C. The Hz13C0 molecules then predissociate or decompose to stable molecular products, HZ and W O . In other examples, vihrations or electronic states of a molecule are excited and a chemical reaction is used to separate the isotopes. The high excitation intensity, narrow line width. and tunability of lasers make them exof all types. cellent sources fo; selective Much new and detailed information is ohtained on the chemistry of excited states. Methods of laser enrichment of fissionable uranium (z35U)have been reported (12). An inexpensive laser separation scheme could have a tremendous economic impact, by reducing the costs of producing energy with nuclear fission. One such scheme is shown in Figure 7. A beam of uranium in a vacuum chamber is crossed with a narrow tunable dye laser and a broad hand ultraviolet source. The dye laser excites an electronic transition in 235U only. The uv lamp is ahle to photoionize the excited z35U,hut not the unexcited 238U.The ions can then he deflected by charged plates and collected. Prohahlv the most ambitious oroied in laser induced . chemistry are the attempts to produce nuclear fusion (13). At first it was thought that strone marmetic fields would he densitieswith a high enough temahle to confine Derature to carrv out controlled thermonuclear fusion. Now ;he use of high powered lasers to produce fusion by implosion appears even more promising. Figure 8 shows a highly simplified conception of a fusion reactor. A numher of high energy (1000 J or more) pulsed laser beams are focused onto an ice pellet of deuterium or deuterium-tritium mixture. The laser pulses are able to heat the sample, ionize it, and compress i t by radiation pressure to temperatures of Volume 53. Number 1, January 1976 / 15
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1OS0Kand densities of 1 0 2 ~ o n s / c cFusion . can then occur by a nuclear reaction such as 2(2H) 3He + n,E~~~~~can he extracted from the fast neutrons released to ~ o w e the r
. The.role of lasers in selective chemistry is still in its infancy. It is likely that lasers will become an even more important part of chemical industry, research, and education. We can be extremely optimistic about the scientific and technological progress which the application of lasers to chemistry will provide in years to come.
16 1 Journal of Chemical Education
Literature Cited 111 L~"KY"I,B.~ . , ' ~ a s e r s : ~ o h n ~ i ~ e y &~~eolnvs~, o r1971. k. 121 Leone. S. R., and Mmrc, C. B., "Laser Sources" in "Chemiea1 md Biolagicd Applicat-soihber~"i~di:or: ~ w r eC. . 6.). A C ~ ~ ~ ~ NC ~ PW Y~ O~, ~19n. S. ~ .
171 Mmm.C. %and Zitte1.P. F..Seience, 182,541 @No". 19731. 18) Alfano, R. R., and Shapiro. S. L., L i . Arnrr., 228.43 (June 1973). (91 B-u, N. G.,orseu~ky.A. N.. and pankratov, A. v., " ~ t i ~of ~chemical ~ ~ RBt i ~ ~ s c t k m with ~ m e Rsdietion." r in ' " c h e m i d and B ~ O I O ~ ~ C ~S I ~ ~ l i ~I.*-~ i i ~ ~ . sers: (Editor Moore. C. B.1, Academic Press,New York, (1974). (10) M r n r e , C . B . , ~ ~Chrrn. ~ . R S S , 6 3 2 3 119731. ('1) P ~ Y * ' cToday. ~ 27 (91, '7 119741. (12) Cham. a n d ~ n g~. e w si,~ u l y8,19741, p. 24. 1131 Emmett,J. L., Nuckolk.J.. W o d , L.,Sci. Amer. 230.24 (June 1974).
